1. Field of the Invention
This invention relates to methods and apparatus for making reflection holograms, and to holograms made thereby.
2. Description of Related Art
During the last few years many optical systems have made use of holographic optical elements to save space, weight or accomplish optical effects impossible by conventional methods. One particular group of holographs used as wavelength selective mirrors are known as reflection holograms. These holograms consist of a recording of a set of interference planes creating, within a suitable recording medium, modulations in refractive index or aborption. These modulations act to reflect only light for which the optical path difference between the planes is a multiple of the wavelength of the incident light. This means, since the path difference is angularly dependent, that the wavelength of the reflected light is also dependent on angle and obeys the Bragg condition: ##EQU1## where .lambda.=Wavelength of light reflected
.theta.=Internal angle of incidence of reflected light to hologram planes
One application of such a hologram is as a laser eye protection filter. In such an application the hologram is required to have the property of reflecting light of the relevant wavelength or wavelengths directed towards the position at which the eye will be located in use.
One known method of making such a hologram will now be described with reference to FIG. 1 which is a diagram illustrating the method.
Referring to FIG. 1, in this method a coherent light beam 1, e.g. from a laser, is formed into a cone of light 3 by a lens 5. The apex 7 of the cone 3 constitutes the so-called construction point of the hologram, corresponding to the position at which the eye will be located in use of the hologram. The hologram is formed in a film 9 of light sensitive material onto which the cone of light 3 is directed. The film 9 is positioned at a distance from the apex 7 corresponding to the desired position of the hologram with respect to the eye in use, the film 9 typically being of spherical form with its concave side towards the construction point 7 as shown in FIG. 1. The film is supported on a substrate 11 of transparent material, e.g. glass, disposed on the same side of the film as the lens 5. Normally a spatial filter 13 is positioned at the apex 7 to prevent skew rays reaching the film 9. The hologram is produced in the film 9 by interference between light directly transmitted through the film 9, and light reflected back through the film 9 after transmission therethrough at the surface of a mirror 15 on the side of the film 9 remote from the source of the light beam. Instead of a mirror surface, Fresnel reflection within the film 9 at the surface of the film 9 remote from the source of the light beam may be used, Fresnel reflection being the reflection of a small fraction of light occurring whenever light is incident on the boundary of a transparent medium. After exposure the film is processed in known manner to fix an image of the interference pattern in the film, as described for example in U.S. Pat. No. 4395088 filed in the names of Kenneth Firth and Richard Keith Howard on the 11th Mar., 1981, which is hereby incorporated by reference.
The above described method of producing a reflection hologram suffers from the difficulty that the energy profile of the light beam needs to be made as uniform as possible, in order that the hologram produced may have uniform spectral properties, and that suitable light sources, e.g. lasers, typically give a beam of gaussian energy profile.